Cardiovascular MRI (CMR) can play a significant role in the diagnosis, staging, and monitoring of numerous ischemic and non-ischemic cardiomyopathies. Recently, parameter mapping in the heart has expanded the CMR toolbox, enabling the detection of diffuse pathologies and quantitative diagnosis. Among quantitative imaging technologies, myocardial T1 mapping has shown promising diagnostic and prognostic value in a wide range of diseases.
Early techniques for quantification of the longitudinal relaxation time (T1) in the heart have employed continuous imaging using equidistant FLASH excitations, following an initial inversion pulse, as originally proposed by D. C. Look and D. R. Locker in “Time Saving in Measurement of NMR and EPR Relaxation Times,” Review of Scientific Instruments, 1970; 41:250-251. This technique allowed regional estimation of the T1 time by evaluating separate regions-of-interest in each cardiac cycle.
For voxel-wise quantification, the Modified Look-Locker Inversion recovery sequence (“MOLLI”) was introduced, which performed single-shot imaging triggered to the end-diastolic quiescence in a Look-Locker type inversion-recovery experiment, as described by D. R. Messroghli, et al., in “Modified Look-Locker inversion recovery (MOLLI) for high-resolution T1 mapping of the heart,” Magnetic resonance in Medicine, 2004; 52(1):141-146. The MOLLI pulse sequence enabled spatially-resolved quantification of the T1 time as a parameter map (T1 mapping) and established widespread use of quantitative CMR. Other imaging sequences, based on inversion or saturation recovery, or a combination of both, have been subsequently introduced for myocardial T1 mapping, each offering a distinct profile of advantages and disadvantages.
Myocardial T1 maps are conventionally acquired at a single end-diastolic phase. Recently, quantitative imaging during systole has been introduced, promising reduced partial volume effects, as well as increased resilience to heart-rate variability. Also, imaging throughout the cardiac cycle using variable flip-angle steady-state acquisitions has been explored for quantitative mapping in a preclinical mouse study, as described by B. F. Coolen, et al., in “Three-dimensional T1 mapping of the mouse heart using variable flip angle steady-state MR imaging,” NMR Biomed, 2011; 24(2):154-162. However, due to limitations in terms of B1+ correction in the sequence proposed by B. F. Coolen, et al., quantitative T1 times were only derived at the end-diastolic phase.
Quantitative analysis of the T1 time in the myocardium conventionally entails manually delineating the myocardium from the surrounding blood pools. Hence, consistent contouring of the myocardium is important for the reproducibility of cardiac T1 measurements. However, sub-epicardial fatty infiltrations, myocardial crypts, trabeculae, and other structures have previously been shown to hamper the identification of the blood myocardial interface in single-phase cardiac images. Better temporal resolution, or the ability to view the changes in these structures over the cardiac phase, may facilitate differentiation.